add admin sidebar layout with responsive drawer navigation

- New admin root + child layouts with daisyUI drawer sidebar
- AdminLayoutHook tracks current path for active nav highlighting
- Split router into :admin, :admin_theme, :user_settings live_sessions
- Theme editor stays full-screen with back link to admin
- Admin bar on shop pages for logged-in users (mount_current_scope)
- Strip Layouts.app wrapper from admin LiveViews
- Remove nav from root.html.heex (now only serves auth pages)
- 9 new layout tests covering sidebar, active state, theme editor, admin bar

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/controllers/admin_controller.ex

@@ -2,6 +2,6 @@ defmodule SimpleshopThemeWeb.AdminController do
   use SimpleshopThemeWeb, :controller
 
   def index(conn, _params) do
-    redirect(conn, to: ~p"/admin/theme")
+    redirect(conn, to: ~p"/admin/orders")
   end
 end